Delhi teeming with dengue virus types

India's national capital is facing the threat of dengue like never before.

Finally, Delhi has been branded 'truly hyperendemic' for dengue. This, after a shocking new research reported last week1 that all the four notorious types of the dengue virus had simultaneously infected people during a single outbreak of the disease in 2006. This is an extremely rare happening, considering that generally one or two serotypes of the virus are at work during an outbreak.

The researchers themselves were taken by surprise at the find. "Multiple dengue virus serotypes have been reported from many parts of the world but concurrent infection with more than one serotype of the virus in one person is rarely documented," Dr Shobha Broor of the Department of Microbiology at All India Institute of Medical Sciences (AIIMS) told Nature India.

Dengue has become a nuisance now

The unusual occurrence came to light when an inter-departmental team from AIIMS started analyzing samples from an outbreak of dengue hemorrhagic fever and dengue shock syndrome (DHF/DSS) that occurred during 2006. In the first such report from India with high percentage of concurrent infections, they have shown that all the four dengue virus serotypes (DENV-1 to DENV-4) were found to be co-circulating in this outbreak with DENV-3 being the predominant one.

Among the 48 samples that tested positive for dengue virus between August and November 2006 at the AIIMS outpatient department, the team found nine samples infected with more than one serotype of the virus. This makes for a very high concurrent infection rate of 19 per cent.

The first case of concurrent infections with two dengue virus serotypes was reported from Puerto Rico in 1982. Previous studies with lower percentage of concurrent infections have been reported from Taiwan (9.5%), Indonesia (11%) and Mexico and Puerto Rico and Indonesia together (5.5%).

From endemic to hyperendemic

Delhi’s journey from being endemic to hyperendemic for the killer mosquito-borne disease is dotted by a series of outbreaks in 1967, 1970, 1982, 1988, 1996 and 2003.

All four dengue virus types circulate in India and cause epidemics, but only occasional cases of DHF/DSS were reported from Delhi till 1996. A large outbreak of DENV-2 occurred in Delhi in 1996. Broor says. "Till 2003, Delhi was hypoendemic for Dengue but the same year all four dengue virus subtypes were found to co-circulate in Delhi thus changing its status to a hyperendemic state."

The team had earlier reported two cases of dual infection with DENV-1 and DENV-3 serotypes in 2006. But now the number has significantly increased. A major concern in such multiple infections is the chance of recombination of these strains that could lead to emergence of more virulent strains. Also, concurrent infections by multiple dengue virus serotypes are known to influence the clinical expression of the disease making it difficult for doctors to diagnose and treat it.

Is India awake to this?

"Dengue has become a nuisance for us now," says the Director General of Indian Council of Medical Research (ICMR) Dr S K Bhattacharya. Public health measures are difficult to implement as the mosquito Aedes Egyptii breeds in unusual places. "The only means to combat this is through creating awareness and vaccination," he insists.

India uses a Chinese vaccine for dengue now. Efforts are on at a couple of national institutes to develop an Indian vaccine but that will take time. Bhattacharya says the National Institute of Virology in Pune is studying the various strains of the virus closely and undertaking major studies relating to diagnosis of the disease.

Navin Khanna, who leads dengue vaccine research at the International Centre for Genetic Engineering and Biotechnology (ICGEB) in New Delhi says dengue is treated in India as the 'poorer cousin of malaria'. "Our government policies lack focus when it comes to third world pathogens," he says. He feels that with co-circulating virus types, India urgently needs a tetravalent vaccine that can provide protection against all four serotypes at one go."But we have so few people dedicated to dengue research in this country that it's going to take a long time to come up with a vaccine unless we decide to put our act together now."

The researchers of this work are from the departments of
microbiology, medicine and pediatrics, All India Institute of Medical
Sciences, New Delhi, India.